This invention relates generally to an improvement over pressure reducing control valves utilized for feeding an aeration fluid stream to a flotation tank.
One of the problems prevalent in dissolved air flotation systems is disturbance of the float blanket by turbulence resulting from rapidly rising large bubbles issuing from the depressurized recycle stream, which has been subjected to air under pressure in order to dissolve the requisite amount of air. The formation of large bubbles appears to result from at least two design characteristics. One is related to the valve or pressure reducing element itself (orifice, nozzle, valve, etc.) and its own design characteristics, and the other relates to other velocity and impingment effects that occur in the pipeline between the pressure reducing elements and the point of introduction into the flotation tank. These velocity and impingement effects cause coalescence of small bubbles into the undesirable larger ones, either through direct contact of one bubble with another, through surface effects, or via some other mechanism.
Thus, with ordinary control valves, a large percentage of the air or gas released on depressurization is in the form of relatively large bubbles, which rise rapidly from the mix chamber or point of entrance of the inlet flow to the top of the flotation tank, sometimes with such velocity and creating such as upward flow velocity of water in the tank that the float blanket above the mix chamber or inlet area is disrupted. This acts in opposition to the function of the dissolved air flotation system in that it makes the skimming and removal of floated solids less efficient, with lower solids levels in the discharged material, and often is so severe that the material which has been floated to form a blanket is re-dispersed and must pass through the action of the system all over again, thereby further reducing the efficiency.